Fracture toughness of coarse-grain heat affected zone of quenched and tempered CrMo steels with internal hydrogen: Fracture micromechanisms

The determination of fracture behaviour of welds in presence of internal hydrogen is essential for the integrity assessment of CrMo vessels and pipes working under high hydrogen pressures. Homogeneous coarse-grain tempered bainitic/martensitic microstructures were obtained in 42CrMo4 and 2.25CrlMo steels by means of laboratory heat treatments in order to reproduce the coarse grain heat affected zone (CGHAZs) of real welds. Afterwards, the fracture toughness behaviour of the simulated CGHAZs was assessed under low displacement rate J-fracture tests, using pre-charged (in hydrogen gas) CT specimens. The hydrogen embrittlement (HE) experienced by the CGHAZs of both steels was considerably greater than in the base steels. The greater hardness, i.e. higher dislocation density, characteristic of the CGHAZ microstructures, explains the more strongly trapped hydrogen, and hence the sharper drop in the fracture toughness, also associated to an increase of intergranular fracture micromechanisms. Scanning electron microscopy examination of the fracture surfaces revealed the action of hydrogen-enhanced localized plasticity (HELP) mediated hydrogen-enhanced decohesion (HEDE) micromechanism, in which HELP took place first, providing enough hydrogen in the process zone to modify the local resistance, and eventually weakening the cohesion of the internal interfaces, HEDE.